1. Field of the Invention
The present invention relates to an optical lighting system which can be used as an original irradiator in a proximity exposure device for printing circuit patterns of a printed-wiring board; an integrated circuit, and the like, onto a substrate. The present invention is also useful in a contact printer and a process camera for printing the image of an original process film onto a machine plate, and the like. More particularly, the present invention relates to an optical lighting system which can uniformly irradiate the total area of a surface to be irradiated in a proximity exposure device and a contact printer (hereinafter these are simply referred to as a contact or proximity exposure system), or can perform irradiation such that the irradiance of an image surface is uniform in a process camera and the like (hereinafter simply referred to as a projection system).
2. Description of the Prior Art
FIG. 1 illustrates a conventional optical lighting system of a contact or proximately exposure system according to "OPTICAL ENGINEERING HANDBOOK" published by ASAKURA SHOTEN in February, 1986, p. 682. FIG. 2 illustrates a conventional optical lighting system of a projection system.
Referring to FIG. 1, the conventional optical lighting system of a contact or proximity exposure system basically comprises a light source LS; a so-called fly's eye lens OS located in the vicinity of the second focal point of an elliptical mirror EM (for conveging light emitted from the light source LS); and a collimator lens CL located between the fly's eye lens OS and a surface P to be irradiated. The fly's eye lens OS is which is a lens which is formed by arranging a plurality of element lenses on a plane perpendicular to the optical axis of the element lenses. Reflection mirrors M.sub.2 and M.sub.3 act to change the advancing direction of the light. The real image of an exit pupil of the elliptical mirror EM is formed in the vicinity of the front focal point of the collimator lens CL through the fly's eye lens OS. The real image of an exit pupil of the fly's eye lens OS is formed on the surface P to be irradiated through the collimator lens CL. According to this optical lighting system, even if a small gap between an original pattern OG, placed on the surface P to be irradiated, and a photosensitive material PM occurs, gradation or misregistration of a printed pattern image can be prevented, since printing is performed by parallel light. Furthermore, irradiance is increased as high as possible by forming the real image of the exit pupil of the fly's eye lens OS, which is secondary light source, on the surface P to be irradiated. Still further, irradiance distribution is unified by correcting the nonuniform quantity of light caused at the second focal point of the elliptical mirror EM, through the fly's eye lens OS. The collimator lens CL having a long focal length is used since it is necessary to unify irradiance distribution on the surface P to be irradiated.
Referring to FIG. 2, the conventional optical lighting system of a projection system comprises: a number of light source lamps 11 arranged in a light source box 10; a light diffusion plate 12 located in front of the light source box 10: and a gradient filter GF located between the light diffusion plate 12 and an original OG. The gradient filter GF is constructed such that the transmittance of light therethrough becomes lower as it approaches the center. The original OG is irradiated through the gradient filter GF so that irradiance in the peripheral portion of the original's surface (i.e., a surface P to be irradiated) becomes higher, whereby irradiance on an image surface P' is unified. Namely, lowering of irradiance in the peripheral portion of the image surface P', which is caused by a projection lens IL in accordance with the "cos.sup.4 .theta. law", is corrected by the gradient filter GF.
In the conventional optical lighting system as hereinbefore described, an area of uniform irradiance on the surface P decreases when the optical path length is reduced to decrease the overall size of the device. Therefore, it is necessary to lengthen the optical path length in order to enlarge the area of uniform illuminance. Concommitantly, the overall size of the device is increased.
On the other hand, in the conventional projection optical lighting system as hereinbefore described, the quantity of light utilized is inefficient since the gradient filter GF is interposed in the optical path and is needed to unify the irradiance of the image surface